The present invention relates to a thermal conduction device in which a heat sink element is held in contact with the surface of a heat generating element so as to transfer heat in a direction from the heat generating element to the heat sink element, and more particularly to the device which lessens a temperature difference at the contact interface attendant upon the transfer of heat. The present invention is well suited to cool electronic components such as semiconductor devices.
A method of eliminating heat from a heat generating element has been known from, for example, U.S. Pat. No. 3,993,123. According to the method, a large number of integrated circuit chips are packaged on a substrate through metal connections for attaining electrical connections, and a heat sink element is mounted on the upper surface of each integrated circuit chip. Heat generated in the chip is transmitted to a cap via the heat sink element, and is carried away out of the system by a coolant flowing through a passage within the cap. Here, when the chip and the heat sink element are fastened by a solder or the like, shear stresses act on the chip and the metal connections at the time of the heat generation of the chip on account of the differences between the coefficients of thermal expansion of the chip and substrate and between those of the heat sink element and cap, and the chip and the metal connections might be damaged. Besides, operations for the maintenance of the chips are not easy. In general, therefore, the method is adopted in which the chip and the heat sink element are held in a mere contact state without being secured. Meanwhile, in this case, to the end of lessening a temperature difference at the contact interface, a gas of high heat conductivity such as helium gas or a liquid such as grease or silicone oil is often packed between the contact surfaces. However, even when the gas or liquid is packed between the chip and the heat sink element, the extent to which the temperature difference can be lessened is limited. More specifically, the chip and the heat sink element do not come into a perfect surface contact because of the warp of the chip, and a considerably great gap is partly formed at the contact portion thereof. It is known that, in general, the gap is on the order of 20 .mu.m. Since the above temperature difference is proportional to the size of the gap between the surfaces confronting each other, the gap forms an obstacle in the case of intending to lessen the temperature difference.
In addition, a contact interface structure which can correct the warp of a chip so as to bring the chip into close contact with a heat sink element has been proposed as disclosed in "1983 Symposium on VLSI technology, Maui, Digest of technical papers, Session 5-6, pp. 60-61 (Sept. 1983)." According to the proposal, a large number of reentrant cavities each having minute structural dimensions of .mu.m order are provided in the surface portion of the heat sink element to come into contact with the chip, and silicone oil is injected into each of the reentrant cavities so as to form a meniscus at an upper position of the cavity. Using the contact interface structure thus constructed, the chip is adsorbed to the heat sink element by the surface tension of the liquid, and the chip can be held in close contact with the heat sink element under the state under which the warp of the chip is cured. Moreover, since the cavities are in the reentrant form, the menisci within the respective cavities are kept stable.
The prior art based on the reentrant cavities has the following disadvantages:
(1) The fabrication of the cavities is difficult, and the structure is not suited to mass production.
(2) The materials of the contact surface in which such cavities can be machined are limited. It is impossible to freely select a material which is advantageous from the standpoint of, for example, the conduction of heat.
(3) Since the root part of a ridge held between the adjacent cavities is very slender, the structure is mechanically fragile and is unsuited to practical use.
(4) A communicating groove for bringing the reentrant cavities into communication so as to equalize the heights of the menisci in the respective reentrant cavities is provided in only one place at a lower position of the reentrant cavities. Therefore, once a void has been formed within the communicating groove, it is held intact, and the communicating groove fails to function. As a result, the liquid cannot move so as to equalize the heights of the menisci in the respective reentrant cavities, and the effect of the reentrant form is lost.
(5) Since each reentrant cavity is in the two-dimensional form, a great force of adsorption cannot be attained.